The simulation paradigm is central to cryptography. A simulator is an algorithm that tries to simulate the interaction of the adversary with an honest party, without knowing the private input of this honest party. Almost all known simulators use the adversary’s algorithm as a black-box. We present the first constructions of nonblack-box simulators. Using these new non-black-box techniques we obtain several results that were previously proven to be impossible to obtain using black-box simulators. Specifically, assuming the existence of collision resistent hash functions, we construct a new zeroknowledge argument system for NP that satisfies the following properties: 1. This system has a constant number of rounds with negligible soundness error. 2. It remains zero knowledge even when composed concurrently n times, where n is the security parameter. Simultaneously obtaining 1 and 2 has been recently proven to be impossible to achieve using black-box simulators. 3. It is an Arthur-Merlin (public coins) protocol. Simultaneously obtaining 1 and 3 was known to be impossible to achieve with a black-box simulator. 4. It has a simulator that runs in strict polynomial time, rather than in expected polynomial time. All previously known constant-round, negligibleerror zero-knowledge arguments utilized expected polynomial-time simulators.

We present session-key generation protocols in a model where the legitimate parties share only a human-memorizable password. The security guarantee holds with respect to probabilistic polynomial-time adversaries that control the communication channel (between the parties), and may omit, insert and modify messages at their choice. Loosely speaking, the effect of such an adversary that attacks an execution of our protocol is comparable to an attack in which an adversary is only allowed to make a constant number of queries of the form “is w the password of Party A”. We stress that the result holds also in case the passwords are selected at random from a small dictionary so that it is feasible (for the adversary) to scan the entire directory. We note that prior to our result, it was not clear whether or not such protocols were attainable without the use of random oracles or additional setup assumptions.

A central focus of modern cryptography is the construction of efficient, "high-level" cryptographic tools (e.g., encryption schemes) from weaker, "low-level" cryptographic primitives (e.g., one-way functions). Of interest are both the existence of such constructions, and also their efficiency. Here, we show essentially-tight lower bounds on the best possible efficiency that can be achieved by any black-box construction of some fundamental cryptographic tools from the most basic and widely-used cryptographic primitives. Our results concern constructions of pseudorandom generators, universal one-way hash functions, private-key encryption schemes, and digital signatures based on one-way permutations, as well as constructions of public-key encryption schemes based on trapdoor permutations. Our proofs are in the model introduced by Impagliazzo and Rudich: in each case, we show that any black-box construction beating our efficiency bound would yield the unconditional existence of a one-way function and thus, in particular, prove P

Starting with the seminal paper of Impagliazzo and Rudich [18], there has been a large body of work showing that various cryptographic primitives cannot be reduced to each other via "black-box" reductions.

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Concurrent general composition relates to a setting where a secure protocol is run in a network concurrently with other, arbitrary protocols. Clearly, security in such a setting is what is desired, or even needed, in modern computer networks where many different protocols are executed concurrently. Canetti (FOCS 2001) introduced the notion of universal composability, and showed that security under this definition is sufficient for achieving concurrent general composition. However, it is not known whether or not the opposite direction also holds. Our main result is a proof that security under concurrent general composition is equivalent to a relaxed variant of universal composability (where the only difference relates to the order of quantifiers in the definition). An important corollary of this theorem is that existing impossibility results for universal composability (or actually its relaxed variant) are inherent in any definition achieving security under concurrent general composition. In particular, there are large classes of two-party functionalities for which it is impossible to obtain protocols (in the plain model) that remain secure under concurrent general composition. We stress that the impossibility results obtained are not “black-box”, and apply even to non-black-box simulation. Our main result also demonstrates that the definition of universal composability is somewhat “minimal”, in that the composition guarantee provided by universal composability (almost) implies the definition itself. This indicates that the security definition of universal composability is not overly restrictive.

We describe efficient protocols for non-malleable (interactive) proofs of plaintext knowledge for the RSA, Rabin, Paillier, and El Gamal encryption schemes. We also highlight some important applications of these protocols: -- Chosen-ciphertext-secure, interactive encryption. In settings where both parties are on-line, an interactive encryption protocol may be used. We construct chosen-ciphertext-secure interactive encryption schemes based on any of the schemes above. In each case, the improved scheme requires only a small overhead beyond the original, semantically-secure scheme...

Recently there has been an interest in zero-knowledge protocols with stronger properties, such as concurrency, unbounded simulation soundness, non-malleability, and universal composability. In this paper,

Zero-knowledge proofs are proofs that are both convincing and yet yield nothing beyond the validity of the assertion being proven. Since their introduction about twenty years ago, zero-knowledge proofs have attracted a lot of attention and have, in turn, contributed to the development of other areas of cryptography and complexity theory.

Abstract. We consider the round complexity of multi-party computation in the presence of a static adversary who controls a majority of the parties. Here, n players wish to securely compute some functionality and up to n − 1 of these players may be arbitrarily malicious. Previous protocols for this setting (when a broadcast channel is available) require O(n) rounds. We present two protocols with improved round complexity: The first assumes only the existence of trapdoor permutations and dense cryptosystems, and achieves round complexity O(log n) based on a proof scheduling technique of Chor and Rabin [13]; the second requires a stronger hardness assumption (along with the non-black-box techniques of Barak [2]) and achieves O(1) round complexity. 1